1. Field of the Invention
The present invention relates to absolute linear encoders for detecting an absolute position. In particular, the present invention relates to an absolute linear encoder which covers a long measurement range and is easy to handle.
2. Description of the Related Art
In order to enable a linear scale to cover a long measurement range, it is necessary to manufacture a long scale. However, in the case of, for example, a photoelectric scale, there are limitations on the length of an exposure device for forming calibrations on the scale, so that it is difficult to manufacture the integral long scale.
Thus, in an incremental scale in which a calibration pattern for detecting relative displacement has repeated continuity, as described in Japanese Patent Laid-Open Publications Nos. 2004-233346, Hei10-18791, Hei6-137899, Hei6-194186, and 2000-55647, it is proposed that a plurality of incremental scales is aligned in the direction of detection, and a continuous sine wave (analog signal in A and B phases) is output in a scale connecting section irrespective of discontinuous calibrations.
Against the foregoing incremental scale, an absolute scale which can detect an absolute position is in practical use. This absolute scale does not need to take the initial action in order to obtain an original position or an reference position on turning power on, though the incremental scale needs it. Thus, the absolute scale is favorable for a multi-head in which many detection heads are attached to a single scale. Also, the absolute scale is able to be used instead of a Hall sensor for detecting a magnetic pole position, so that it is favorably used for feedback control of a linear motor.
In the case of the absolute scale for detecting the absolute position, however, manufacturing a long integral absolute scale over, for example, 3 m is not easy. Because phase relation differs in each position of calibrations in which, for example, a plurality of continuous patterns at regular intervals is combined in parallel and a calibration pattern does not have repeated continuity. Even if a long scale can be manufactured, there is a problem that the integral scale is not easy to transport and attach.